Sensing an electric parameter such as current and voltage is necessary in most of systems. For example, referring to FIG. 1, a buck power converter 10 uses a pulse width modulation (PWM) controller 12 to switch power MOSFETs M1 and M2 to provide a voltage Vo and a current Io for a load 14, and for regulating the output voltage Vo, the buck power converter 10 needs to sense the inductor current IL. An intrinsic lossless current sensing method is popularly used to sense the inductor current IL, which employs the winding resistance of the inductor L or the on-resistance of the power MOSFET M1 or M2 as the current sensor. However, both the winding resistance of an inductor and the on-resistance of a power MOSFET are temperature dependent, and thus the sensed current signal will not be linearly proportional to the inductor current IL, resulting in poor performance at active voltage positioning (AVP) and over current protection (OCP). Since the intrinsic element value, i.e. the winding resistance of the inductor L and the on-resistance of the power MOSFET M1 or M2, varies with temperature, thermal compensation for the intrinsic element is typically employed to improve the accuracy of the sensed current signal. Therefore, it is required an accurate and low-cost method for sensing the temperature of a power component, such as the power MOSFET M1 and M2 and the inductor L shown in FIG. 1.
Generally, a negative temperature coefficient (NTC) thermistor is used to sense the temperature of a power component for thermal compensation. For example, as shown in FIG. 2, an NTC thermistor 6 is placed nearby the inductor L to sense the temperature of the inductor L, and for precise thermal compensation, the NTC thermistor 16 is required to be as close to the inductor L as possible. According to NTC thermal compensation, U.S. Pat. Nos. 6,833,690 and 6,998,827 disclosed two different thermal compensation methods to compensate the environment temperature variation to improve the accuracy of the intrinsic current sensing method and AVP. However, as a system is operating, the intrinsic element value is varied with its operational temperature, mainly caused by the environment temperature and the power loss of the intrinsic element, and unfortunately, the NTC thermistor can not exactly sense the temperature variation caused by the power loss of the intrinsic element. For example, FIG. 3 shows the temperature measurement at the inductor L and the NTC thermistor 16 versus the output current Io in a system as shown in FIG. 1, in which the curve 18 represents the temperature of the inductor L, and the curve 20 represents the temperature of the NTC thermistor 16. As clearly shown in FIG. 3, the NTC thermistor 16 does not exactly sense the operational temperature of the inductor L. While the NTC thermistor 16 can sense the environment temperature outside the inductor L, the power loss of the inductor L can increase the internal temperature of the inductor L and thus makes the internal temperature of the inductor L higher than the external environment temperature. The NTC thermistor 16 only senses the external environment temperature of the inductor L and part of the internal operational temperature of the inductor L. Therefore, the temperature variation caused by the power loss of the intrinsic element is not precisely sensed and compensated by conventional methods.
The inductor L and the power MOSFETs M1 and M2 are all power components, whose power loss causes considerable temperature variation. Therefore, it is desired an improved intrinsic current sensing method.